Atmospheric fluidized bed combustors are known in the art for combusting fossil fuel materials such as coal. Problems have been encountered, however, in improving the environmental performance of fluidized bed combustors by, for example, minimizing SO.sub.2, NO.sub.x and CO emissions, minimizing the formation of N.sub.2 O and reducing solid waste products through a decrease in sorbent demand.
Although fluidized bed combustors are capable of the simultaneous control of both SO.sub.2 and NO.sub.x, newly introduced emission limitations are pushing the technology toward the limits of economic SO.sub.2 removal. Fluidized bed combustors require a relatively low combustion temperature, typically ranging from approximately 1500.degree. to 1600.degree. F., to optimize sorbent utilization. At these temperatures, fluidized bed combustors require calcium-to-sulfur molar ratios ranging from approximately 1.5 to over 2.5 to achieve SO.sub.2 reductions of approximately 70% to 90%, depending on the type of coal and emission targets. Fluidized bed combustors can achieve greater than 95% SO.sub.2 removal with in-situ sulfur retention or removal chemistry, but require calcium-to-sulfur molar ratios in excess of 3.0 to do so.
The high sorbent requirements in reducing SO.sub.2 emissions in fluidized bed combustors beyond those obtainable under optimal temperatures result in less efficient conversions and solid waste production of approximately 500 lbs for each MBtu of high sulfur coal combusted. In comparison, a pulverized coal fired boiler having a flue gas desulfurization system and operating under similar emission targets produces only approximately 375 lbs for each MBtu generated from the combustion of high sulfur coal.
The relatively low combustion temperatures required to maintain efficient sulfur capture also affect other emissions, including N.sub.2 O, NO.sub.x and CO. Although emissions from combustors are currently unregulated, they may be a target for regulation in the future since N.sub.2 O is considered a green house gas. The amount of N.sub.2 O generated from a combustion process generally decreases with increased combustion temperature. For example, the amount of N.sub.2 O generated from the combustion of high sulfur coal at 3% O.sub.2 is approximately 130 parts per million at 1500.degree. F. and approximately 90 parts per million at 1600.degree. F.
The relatively low combustion temperatures of atmospheric fluidized bed combustors result in approximately 0.25 lb. NO.sub.x for each MBtu generated from the combustion of coal. The amount of CO generated from atmospheric fluidized bed combustors at these temperatures is generally less than 100 parts per million. CO emissions generally decrease with increased combustion temperature.